Engines may be configured to operate with a variable number of active or deactivated cylinders to increase fuel economy, while optionally maintaining the overall exhaust mixture air-fuel ratio about stoichiometry. Such engines are known as variable displacement engines (VDE). Therein, a portion of an engine's cylinders may be disabled during selected conditions defined by parameters such as an engine speed/load window, as well as various other operating conditions including operator torque demand. Conventional VDE control systems may disable a selected group of cylinders, such as a bank of cylinders, through the control of a plurality of cylinder valve deactivators that affect the operation of the cylinder's intake and exhaust valves, or through the control of a plurality of selectively deactivatable fuel injectors that affect cylinder fueling. Newer skip-fire or rolling VDE systems may be configured to activate/deactivate individual cylinders on an ongoing basis to provide a specific firing pattern based on a designated control algorithm.
VDE systems may utilize various methods of torque monitoring for ensuring proper torque delivery and diagnosing potential degradation of the VDE system. Example attempts to address torque monitoring include comparing two independent sources of torque estimates with the torque requested by an operator. One example approach is shown by Light et al. in U.S. Pat. No. 6,705,286. Therein, a torque monitoring algorithm compares driver torque demand to two independent torque estimates, one of which is estimated from throttle position, and the other estimated from mass airflow (MAF) to the intake manifold. If one of the two actual torque estimates exceeds the driver-demanded torque, the monitoring algorithm logic intervenes in engine torque production and sets a diagnostic code. Still other approaches of torque monitoring include fuel-injection based methods, where valve degradation may be assessed based on the duration of a spark event.
However, the inventors herein have recognized potential issues with such systems. As one example, fuel injection-based torque monitoring may not be suitable for use with rolling VDE systems because the number of active cylinders may often be overestimated, and packaging constraints may limit the feasibility of including fuel injector sensing for all cylinders individually. As another example, MAP based approaches for torque estimation rely on a comparison of air flow metering, as determined via a MAF sensor output to the dynamic effect of inducting cylinders, as determined via a change in MAP. However, reliance on MAF sensors for air flow metering and MAP sensors for manifold pressure change adds cost and complexity to the vehicle system. In addition, the sensors may themselves be prone to degradation. Still other approaches for air flow metering may model air flow across a throttle body at selected engine operating conditions. However, such models may be inaccurate at or near wide open throttle (WOT) conditions. As a result of the inaccuracies, the actual torque provided by the VDE system may exceed the driver demanded torque, resulting in a loss of fuel economy as well as drivability issues that are objectionable to the driver. For example, the vehicle may appear “jumpy”. The inaccuracies in air flow estimation can also lead to an actual induction ratio being incorrectly estimated and a VDE mechanism being incorrectly indicated as degraded. If the VDE mechanism is disabled responsive to the indication, engine run time in the VDE mode is unnecessarily reduced, causing a loss of fuel economy.
The inventors herein have recognized that during conditions when the engine is operating at or near wide open throttle (WOT), torque inaccuracies may be reduced by constraining the induction ratio. For example, the VDE system may be limited to operating at an induction ratio of 1 to reduce over-torque issues. The resultant temporary drop in fuel economy may be acceptable to a vehicle driver in view of the improvement in vehicle drivability. Thus, in one example, the above issues may be at least partially addressed by a method comprising operating a variable displacement engine with an induction ratio based on operator torque demand; and responsive to throttle position being within a threshold distance of wide open throttle, independent of the operator torque demand, increasing the induction ratio. In this way, a more accurate torque estimation may be used to more reliably infer the actual induction ratio of a VDE system.
In one example, a variable displacement engine may be operated with one or more engine cylinders selectively deactivated responsive to operator torque demand. For example, at higher engine speeds and loads, the engine may be operated at a higher induction ratio with fewer cylinders deactivated, while at lower engine speeds and loads, the engine may be operated at a lower induction ratio with more cylinders deactivated. If the desired induction ratio is within a threshold of 1.0, the induction ratio applied may be constrained to 1.0 while overriding the desired induction ratio. In addition, during engine operating conditions where the desired induction ratio is outside the threshold of 1.0, but the intake throttle is at or within a threshold distance of WOT, the induction ratio applied may be increased towards 1.0 (e.g., step-wise increased towards 1.0 by incrementally moving to a next highest possible induction ratio) while overriding the desired induction ratio. By constraining the applied induction ratio during conditions when air flow metering may be inaccurate, torque inaccuracies may be reduced. During other conditions, an actual induction ratio may be inferred via a throttle body model used to infer air flow and a rate of change in manifold pressure across the throttle. If the actual induction ratio significantly differs from the commanded induction ratio, degradation of the VDE system may be inferred, and mitigating actions may be taken.
In this way, by relying on a throttle body model for metering air flow, the reliance on a MAF sensor is reduced, without compromising the accuracy of torque estimation or identification of potential VDE degradation. The technical effect of selectively constraining the induction ratio applied during operating conditions where air flow estimation and torque estimation may be inaccurate, the likelihood of over-delivering torque is reduced. By increasing the reliability of actual induction ratio estimation, the likelihood of incorrectly diagnosing VDE degradation and prematurely disabling VDE operation is reduced. By enabling VDE operation over a larger duration of engine operation, fuel economy and engine performance benefits of VDE may be extended.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.